The invention relates to a method for bringing about a mixed state of at least two fluids in a second part of a line that have flowed through a first part of this line, to a use of the method in a crossflow filtration system, and to an apparatus for performing the method.
Known crossflow filtration systems are embodied as multipass systems, in which a plurality of filtration modules are disposed in a plurality of filtration routes that are supplied in parallel. Such filtration routes are known as passes. These passes are fed in parallel with the fluids to be filtered from a distributor line. Each filtration module in turn includes a number of membrane tubules that carry filtration membranes and are subjected in parallel to the fluids to be filtered.
FIG. 1 shows a diagram of one such known crossflow filtration system. It includes nine passes 1. Each of which has four filtration modules 2. In each pass 1, the filtration modules 2 are connected in series with one another. The nine passes are supplied in parallel with fluids to be filtered through a distributor line 3. In the filtration modules 2, a fraction of the fluids is separated out in the form of permeate or filtrate, while the remaining fraction of the fluids is collected as retentate by a collecting line 4 and removed. The removal of the permeate is not shown here.
FIG. 1 shows two fluids 5 and 6 in the distributor line 3, with a parting boundary 7 between them. The fluids 5 and 6 occur when the retentate 6, after the conclusion of a filtration cycle, is positively displaced out of the filtration system by means of water. In the state shown, some of the filtration modules 2 and part of the collecting line 4 are already filled with water 5 as a consequence of the positive displacement, while others of the modules 2 and another part of the line 4 are still filled with retentate 6. Under these circumstances, it is known that individual membrane tubules or entire filtration modules 2 repeatedly become clogged, because the high-viscosity retentate 6 is no longer positively displaced out of the remaining modules 2 if the water 5, which has low viscosity, can flow out through some of the modules that have already been rinsed out.
If the pressure drop through the filtration modules 2 between the inlet 8 of the distributor line 3 and the outlet 9 of the collecting line 4 was still 5 bar, for example, before the water 5 was supplied, then in the state shown in FIG. 1, after rinsing out of the first pass 1, the pressure drop is reduced to approximately 3 bar, and it drops further as further passes 1 are rinsed out. The reduced pressure drop slows down the flow speed in the modules 2 that are still filled with retentate 6. A structural effect increases the high viscosity of the retentate 6 still further, until the flow comes to a stop. The remaining pressure drop of less than 3 bar is then no longer sufficient to positively displace the remaining retentate 6.
Known apparatuses have abated this problem by means of symmetrical distributors and/or very slowly opening water valves while the retentate supply is still open. In the first case, the parting boundary 7 reaches all the passes 1 at the same time, while in the second case the difference in viscosity of the fluid mixtures reaching the passes 1 at the same time is decreased.
Known symmetrical distributors are designed for only a maximum of four passes, for reasons of space and expense, and are usually combined with static mixers. Water valves that open slowly require regulating devices to allow them to reach a sufficiently slow reduction in the viscosity of the retentate. Idle flow zones that nevertheless remain can still cause clogging in this case, however.
Experience shows that despite the known provisions described, clogging of modules cannot be avoided. The situation is especially problematic in filtration systems with distributors for up to two hundred membrane tubes, which is equivalent to a standard industrial-scale system with ten passes.
It is therefore the object of the invention to bring about a mixed state of at least two fluids in a line that makes it possible for module clogging of the kind described to be effectively prevented.
According to the invention, in a method of the kind referred to at the outset, this object is attained in that at at least one point of the second part of the line, at least a fractional flow is withdrawn from the line, and that this fractional flow is returned to the second part of the line again after a time lag. The method is preferably embodied such that the fractional flow is returned to the second part of the line at the same point where it was withdrawn or at a point located upstream thereof.
As an apparatus for mixing fluids in a line, at least one side line is preferably used, which connects at least two points of the line, which are spaced apart from one another in the direction of the line, to one another and recirculates a fractional flow from the downstream line point to at least one of the upstream line points.
A use of the method of claim 1 for avoiding clogs in modules in a crossflow filtration system, in which at least two filtration routes (passes) that include modules are supplied in parallel with the fluids to be filtered from a distributor line, is distinguished in that the second part of the line is used as the distributor line for the passes.
Further variants of the method as well as its use and the apparatus for performing it are defined by the claims.